1. Field of the Invention
The invention relates generally to an embedded access point, in particular an embedded access point for use in association with mobile units in a wireless communications network.
2. Description of the Related Art
Wireless communication networks such as wireless local area networks (WP) are used in many applications such as inventory, package tracking, portable point of sale and so forth. Generally the operator will carry a mobile unit such as a hand-held computer which communicates with a host computer via one of a plurality of access units (or access points) connected to the host computer. Many of the systems that have been developed are proprietary and in order to achieve inter-operability of the various systems a standard IEEE 802.11 has been established.
The concept of "roaming" mobile units has also been addressed in the prior art. Where a mobile unit is portable and communicates via, for example, radio frequency communication, the unit may be transported out of range of a given access unit or at least to a location where it is within range of more than one access unit. In either case it is desirable for the mobile unit to have the option of selecting which access unit it should associate with on the basis of the strength of signal received from different access units.
The IEEE 802.11 protocol supports either direct sequence or frequency hopping spread spectrum systems, as well as infrared communications. For the purposes of the present discussion, frequency hopping spread spectrum communication will be concentrated on. Each access unit executes a unique hopping pattern across a given number, conventionally 79, of non-overlapping frequencies at a rate of one hop every one hundred milliseconds. Sixty six hopping patterns are specified in the IEEE 802.11 protocol and are selected to minimise the possibility of interference.
In order for a mobile unit to associate with an access unit the mobile unit follows an association protocol. The mobile unit firstly sends out a probe packet having no destination address which is accordingly accepted by all access units within range. The probe packet contains an identifying address for the mobile unit which has sent it. The access unit then transmits a probe response packet which includes information such as the access unit address, the hopping pattern, the present channel, time left in the present channel and other timing information. The mobile unit then decides whether or not to associate with a given access unit, based on for example the strength of the signal of the access unit and any information the access unit may have issued indicating how many mobile units are already associated with it. If the mobile unit decides to associate, it sends an associate message or packet and the access unit decides whether to accept the association request and issues an association response after the request is accepted.
In addition the access unit transmits a "beacon" at predetermined intervals containing, in addition to other information, timing information similar to that contained in the probe response packet.
The mobile units can operate in two power management modes, either continuously awake mode (CAM) or power save polling (PSP) mode. In the former mode, CAM, the mobile unit remains in substantially continuous communication with an access unit so as to receive and transmit all information intended for the mobile unit practically instantaneously. Of course that mode of operation requires a high level of power consumption which is not always desirable for a portable mobile unit which is relying on internal power such as batteries. In the alternative PSP mode the mobile unit sends out a polling signal at predetermined intervals of time to enquire whether an associated access unit has stored any messages for that mobile unit in a suitable buffer. Similarly the mobile unit can store any messages to be transmitted in a buffer and transmit all of the messages so stored at predetermined intervals. Such a mode of operation clearly allows decreased power consumption. Under the IEEE 802.11 protocol the beacon signal contains information about which PSP stations have data waiting.
A standard access unit or access point (AP) can support wireless communication with up to 128 mobile units. The standard AP also communicates with a wired Ethernet backbone, performing a bridging function between the wired and wireless sides. The standard AP has a serial port to allow monitoring of network operation.
The conventional system is highly efficient and reliable, but in certain cases may prove very costly for the desired application. For example certain configurations require a very small number of mobile units (MU), perhaps one or two, and a data transfer rate which is correspondingly low, perhaps 10 to 20 KB/sec. These configurations include truck based systems and low end non-mall configurations such as convenience stores. In these environments the system described above including a separate access point wired to a host computer introduces a level of cost and complexity which is not required for the simple applications envisage with a very small number of mobile units.
One solution that has been proposed is to operate in a "ad-hoc" mode. This is an approved IEEE 802.11 mode of operation in which there is no AP. Instead one of the MU's takes on the burden of generating beacons to coordinate the hopping sequence among the various MU's. The major disadvantage to this approach is that support for a power saving mode (PSP) is not present; instead the system requires that all MU's are operating in CAM. Such an arrangement is not always desirable for hand-held terminals.